Manufacture of strings



July 5, 1955 J. T. CRANDALL MANUFACTURE OF STRINGS Filed Sept. 7, 1949 INVENTOR.

ATTORNEY United States Patent OfiFice 2,712,263 Patented July 5, 1955 MANUFACTURE OF STRINGS Julian T. Crandall, Ashaway, R. I. Applicatinn eptembcr 7, 1949, Serial No. 114,398

6 Claims. (Cl. 87-1) The present invention relates to the manufacture of strings for stringing tennis, badminton and squash racquets and for musical instruments, and has particular reference to a novel manufacture and a novel construction therefor.

The principal object of the invention is to provide an improved string for tennis racquets, musical instruments, and the like.

Another object of the invention is to provide an improved string comprising a filament core and an integrally joined sheath.

A further object of the invention is to provide an improved string having great strength, elasticity, and resistance to fatigue and distortion.

With the above and other objects and advantageous features in view, the invention consists of a novel method and a novel article more fully disclosed in the detailed description following, in conjunction with the accompanying drawings, and more specifically defined in the claims appended thereto.

In the drawings,

Fig. 1 is a perspective view of a tennis racquet strung with the improved string;

Fig. 2 is an enlarged portion of the string, parts being broken away to show the elements thereof; and

Fig. 3 is a perspective enlarged view showing the appearance of the string during the different stages of manufacture.

It has been found desirable to provide an improved string for tennis racquets and the like, which has great strength, is very resilient, has great resistance to tension strains and to flexing, and has a smooth water repellent outer surface. To this end, I have devised a composite construction which utilizes a center flexible core of high resiliency, preferably made of twisted synthetic plastic filament, and a braided cover which is a tough armor sheath, preferably made of synthetic plastic filament, the cover being integrally locked to the core to provide a smooth, hard, abrasion resisting string.

I have found that a suitable plastic for such strings is of the amido-polymer type, and that nylon is particularly desirable because it orients under tension when drawn down to the desired diameter and increases the strength of the string.

Referring to the drawings, Fig. 1 illustrates a racquet which has been strung with the improved string 11. The

string 11, as shown in Fig. 2, has a twisted core 12 which is flexible and of high resiliency and a braided armor sheath 13, which is tough and wiry, the core and the sheath being integrally locked, preferably with synthetic plastic 14. The resulting string has an integrated twisted core and a braided sheath, is hard and tough, and does not abrade when pulled through the string holes of the racquet.

The improved string is manufactured by first forming a line of twisted strands as indicated by the reference numeral 15 in Fig. 3. This line is passed through a tank containing a solution of a special coating material, which preferably has a nylon base, it being preferred to apply several thin layers of a coating rather than a single heavier layer. Between the coats the line is lightly brushed to obtain a smooth finish, as indicated by the reference numeral 16. The coated line is then air dried, and a sheath, which is preferably of nylon monofilament thread, is braided over the coated line as indicated by the reference numeral 17. The covered line is again coated by passing through the integrating solution, preferably to obtain several thin layers, and is lightly brushed between each coating to provide a smooth coated braid covered line as indicated by the reference numeral 18. The coated braid covered line is now stretched under tension and under predetermined heat conditions, whereby the line is reduced in diameter to form a completely integrated string, with increased tensile strength, as the two coatings soften under the heat and pressure to lock the parts together. The stretched string, indicated by the reference numeral 19, is now quickly cooled, either by air or by passing through a water bath, and the resulting product is a strong, completely integrated string having a twisted highly resilient core with a braided armor sheath. The number and extent of the coatings are regulated to obtain a surface which will have a desired effective bite on the ball when in play. When the string is manufactured for musical instruments, the number and extent of the coatings are regulated to have a desired tone.

Although I have found that synthetic resin solutions may be used for integrating the string, the preferred coating solution includes nylon in flake form, dissolved with isopropanol and tetrahydrofurfuryl alcohol in a water base, the proportion of nylon flake being variable between the limits of 10 to by weight, of isopropanol between the limits of to by weight, and of tetrahydrofurfuryl alcohol between the limits of 5 to 20% by weight, the proportions being chosen and sufficient water being added to provide a coating solution of the desired consistency.

The nylon flake, which is of shredded type, is added to the other ingredients under heat to'form the coating solution, and the proportions of the ingredients are varied to provide a desired stiffness in the final product. For strings requiring more flexibility, it isdesirable to add a predetermined amount of plasticizer. It has been found that a string which has a diameter of .056 inch before integration will break under a dead load of one hundred pounds; after integration, with a diameter reduced to ,046 inch, the break load has risen to one hundred ten pounds.

The integration drawing for a string of the size described is preferably under a strain load of fifty pounds,

using a drawing speed of about eight hundred yards per hour and a temperature of substantially 360 F. The resulting string has the stiffness suitable for use in tennis racquets and the like. Change in drawing speed, pull and temperature will produce a variation in the stiffness; the range of speed is from two hundred to one thousand yards per hour, and the range of temperature is from 250 to 400 F.

A string made as described has high tensile strength, very high resiliency when strung, and great resistance to fatigue and distortion, whereby the racquet strings hold their shape, give long wear, are water repellant, and do not elongate or strech with continued use. When the string is made of nylon, molecular orientation is produced by the stretching and is fixed by the quick cooling. Al though the core is preferably twisted to provide high resilience, it may be desirable for certain purposes to use a multiple braided core or a multiple linear filament core.

The preferred material for the novel string is nylon, but other suitable material may be used for composite strings for different uses, with textile and silk cores or sheaths, and in combination with plastic materials, suitable binders being utilized to integrate a core of flexible material with a cover or sheath of tough material.

Although I have described a specific string construction designed for racquets and musical instruments, it is obvious that the string may be used for other purposes, such as for example for fish-hook leaders, and fishing lines, and that changes in the size and shape of the parts and in the material used may be made without departing from the spirit and the scope of the invention as defined in the appended claims.

I claim:

1. In the manufacture of strings, the steps of forming a twisted core of thermo-plastic filaments, coating the core with thermo-plastic material, drying, braiding an armor sheath of thermo-plastic filaments around the coated core, coating the braided article with thermo-plastic material, and integrating the core, the sheath and the coatings by heating under tension and then cooling, the thermoplastic material of said core, braided sheath, and coatings having similar physical and chemical properties.

2. In the manufacture of strings, the steps of forming a twisted core of thermo-plastic filaments, coating the core with thermo-plastic material by passing the core through a solution of the thermo-plastio material, drying, braiding an armor sheath of thermo-plastic filaments around the coated core, coating the braided article with thermoplastic material by passing the braided article through a solution of the thermo-plastic material, and integrating the core, the sheath, and the coatings, the thermoplastic material of said core, braided sheath, and coatings having similar physical and chemical properties.

3. In the manufacture of strings, the steps of forming a linear core of thermoplastic material, coating the core with thermoplastic material in solution, drying, braiding thermoplastic filaments around the coated core to form an armor sheath, coating the sheath and the coated core with thermoplastic material in solution, and stretching the sheath and coated core under heat and tension to integrate the core, sheath, and coatings, the thermoplastic material of said core, braided sheath, and coatings having similar physical and chemical properties.

4. In the manufacture of strings, the steps of forming a core of a plurality of adjacent nylon filaments, coating the core with nylon material in solution, drying, braiding nylon filaments around the coated core to form an armor sheath, coating the sheath and the coated core with nylon material in solution, and stretching the sheath and coated core under heat and tension to integrate the core, sheath, and coatings, the thermoplastic material of said core, braided sheath, and coatings having similar physical and chemical properties.

5. A string having a core of thermoplastic material and a sheath around said core of braided thermoplastic filaments, said sheath and said core being bonded with thermoplastic material, said sheath, material being of the amido polymer type and integrated and oriented by stretching under heat and tension.

6. A string having a core of adjacent filaments of nylon material and a sheath around said core of braided nylon filaments, said sheath and said core being bonded with nylon material, said sheath, core, and bonding material being integrated.

References (Zited in the file of this patent UNiTED STATES PATENTS 375,952 Requa Jan. 3, 1888 2,091,999 Madge et al Sept. 7, 1937 2,308,825 Rawlings Jan. 19, 1943 2,322,756 Wallder June 29, 1943 2,327,104 Gudebrod Aug. 17, 1943 2,478,940 Pape Aug. 16, 1949 2,500,332 Whitehead Mar. 14, 1950 2.649.833 Crandall Aug. 25. 1953 FOREIGN PATENTS 552,160 Great Britain Mar. 25, 1943 606,107 Great Britain Aug. 6, 1948 645.203 France June 26. 1928 core, and bonding 

